<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="UTF-8">
  <meta http-equiv="X-UA-Compatible" content="IE=edge">
  <meta name="viewport" content="width=device-width, initial-scale=1.0">
  <title>Document</title>
</head>
<body>
  <div id="app">
    <button @click="addLi">新增li</button>
    <ul id="oul">
      <li v-for="item in arr" :key="item">
        {{item}}
      </li>
    </ul>
  </div>
  <script src="./vue.js"></script>
  <script>
    const vm = new Vue({
      el: '#app',
      data: {
        arr: [1,2,3,4]
      },
      methods: {
        addLi(){
          const oul = document.querySelector('#oul');
          this.arr.push(this.arr.length+1);
          this.$nextTick(()=>{
            /* 
            在每一次数据改变后，更新最新dom完毕后出发
            */
            console.log(oul.children);
            oul.lastElementChild.style.background="red";
          })
        
          // oul.lastElementChild.style.background="red";

        }
      }
    })
  </script>
</body>
</html>